The present invention relates generally to high speed optical modulators. More particularly, the present invention relates to a high speed optical modulator having an integrated driver amplifier.
Many high frequency electronic components, devices, and subsystems, e.g., those utilized in radio frequency (RF) and microwave applications, are designed such that they exhibit a standard input/output port impedance of 50 ohms. Although the standard 50 ohm impedance provides connection uniformity and reduces the amount of customized system design, it can restrict the design and/or performance of certain components and subsystems. High speed optical modulators are no exception and their 50 ohm RF input port impedance is normally compatible with driver amplifiers having a 50 ohm RF output port impedance.
The prior art is replete with various RF and microwave driver amplifier components and optical modulator components. Briefly, such driver amplifiers receive an electrical data signal having a high data rate, e.g., 10 Gb/s. The data signal propagates over a transmission line having a standard line impedance of 50 ohms. Thus, most prior art driver amplifier components utilize 50 ohm input and output connectors that match the 50 ohm transmission line impedance. In addition, such driver components utilize amplifier chips or devices having 50 ohm input and output impedances.
Many high speed systems employ optical modulators, such as the Mach-Zehnder optical modulator. Mach-Zehnder optical modulators are well known to those skilled in the art and, therefore, are not described in detail herein. Briefly, an optical modulator can be configured to receive an amplified data signal from a driver amplifier component, along with an unmodulated continuous wave (CW) optical input. The optical modulator is configured to modulate the optical signal in response to the electrical data signal. The optical input is carried by an optical waveguide formed within a substrate, and the electrical data signal propagates over a transmission line located above the optical waveguide. Most prior art optical modulators utilize a 50 ohm transmission line that is compatible with the 50 ohm output impedance of most standard RF driver amplifier components.
Prior art driver-modulator combinations have required the driver output impedance and modulator input impedance to be 50 ohms for compatibility with the standard for external connectors and for standard chip interfaces. Mach-Zehnder optical modulators having a 50 ohm RF input impedance require a relatively high electrical driving voltage (this driving voltage is typically known as the Vxcfx80 of the modulator). Unfortunately, most practical driver amplifiers cannot achieve the required drive voltage, particularly at very high data rates of up to 40 Gb/s. For example, conventional Mach-Zehnder modulators having 50 ohm impedances operate with a Vxcfx80 of approximately six to eight volts. However, currently available driver amplifiers can only achieve approximately five volts at 40 Gb/s. In addition, higher Vxcfx80 values result in higher driver gain requirements and correspondingly higher driver operating temperatures. Due to the high temperature sensitivity of practical Mach-Zehnder modulators, excessive heat generated by the driver can adversely affect modulator performance, especially if the modulator is not thermally insulated from the driver. Accordingly, lower Vxcfx80 values may lead to better modulator performance.
One way to decrease the Vxcfx80 of the modulator is to reduce the characteristic impedance of the modulator transmission line. However, reducing this impedance affects the impedance of the RF modulator port unless a matching circuit is implemented between the driver output port and the modulator transmission line. Unfortunately, matching circuits are not desirable in most practical high speed applications because they adversely affect the frequency response of the modulator and increase the insertion loss and length of the modulator.
An integrated driver/modulator assembly according to the present invention effectively reduces the Vxcfx80 requirement of the optical modulator such that the assembly can utilize practical driver amplifier designs in high speed (e.g., 40 Gb/s) applications. In accordance with the preferred embodiment, the driver/modulator assembly utilizes an electrical transmission line having a characteristic impedance that is less than the standard 50 ohm impedance found in most commercially available RF and microwave components. In addition, the driver/modulator assembly preferably employs a driver amplifier design having a final output impedance that matches the reduced impedance of the modulator transmission line. In this regard, the driver/modulator assembly need not utilize an impedance matching circuit between the output of the driver amplifier and the input of the optical modulator. Furthermore, the driver/modulator assembly need not utilize external RF connectors between the driver amplifier and the optical modulator; the preferred practical embodiment houses the driver amplifier and the optical modulator together in a single component package.
The above and other aspects of the present invention may be carried out in one form by an integrated driver/modulator that includes a driver amplifier assembly having a driver input impedance and a driver output impedance different than the driver input impedance, and an optical modulator coupled to the driver amplifier assembly. The optical modulator includes an electrical transmission line having a modulator transmission line impedance that matches the driver output impedance.